High temperature high strength alloys



.' P tented Mar. 4, 1952 HIGH TEMPERATURE HIGH STRENGTH ALLOYS Peter Payson, New York, N. Y., assignor to Cru- V cible Steel Company of America, New York,

N. Y., a corporation of New Jersey No Drawing. Application December 2, 1948, Serial No. 63,200

6 Claims. (01. 75 12s This invention pertains to a forgeable alloy steel having high strength at high temperatures, and more particularly to an alloy steel of this character which is characterized by high resistance to rupture under stress at temperatures up to about 1500 F.

The invention provides a new and improved corrosionand oxidation-resistant, forgeable alloy steel of this type, having a 100-hour stress-torupture value at 1500 F. of at least 15,000 pounds per square inch (p. s. i.), and ranging upward therefrom to about 20,000 p. s. i., and which achieves this result without necessity for cobalt, columbium or titanium additions. In fact, the alloy steel of the present invention contains, in its preferred embodiment, no cobalt, although it optionally may include small amounts of columbium, up to about 1%, or titanium, up to about 2%.

The new forgeable alloy steel of the invention is a precipitation or age-hardening, austenitic alloy steel, containing as essential constituents: about 20 to 30% nickel; about 16 to 22% chromium; about 3 to each of tungsten and molybdenum, the sum of these two elements being, however, held within limits of about 10 to Within the limits aforesaid, the chromium content in the steel is always less than the nickel content, for maintaining the austenitic structure and for avoiding the formation of delta ferrite. 9

Carbon is present in the steel for purposes of stabilizing the austenitic structure, ranging from effective amounts up to about 0.3 the upper limit being required for retention of forgeability of the steel. Manganese and silicon are both present in the steel in usual amounts ranging up to about 2% for manganese and up to about 1% for silicon. As above stated, the steel may contain either or both titanium and columbium in small amounts, titanium up to about 2% and columbium up to about 1%, although neither of these elements is required for achievement of the stress-to-rupture values aforesaid. On the same basis, vanadium may be present in the steel up to about 3%. Also, as above stated, cobalt is not an essential constituent of the steel and is ordinarily not present.

A preferred range of analysis of steel in accordance with the present invention is the following: carbon about 0.1 to 0.2%; nickel about 22 to chromium about 16 to 22%; tungsten and molybdenum, 3 to 8% each, with the sum of the two limited, however, to about 10 to 16%; manganese about 1 to 2%; silicon about 0.2 to 0.5%; and the balance substantially iron, except for optional 2 additions of small amounts of any or all of vanadium, titanium and columbium, each within limits as above stated.

The alloy steels of the invention are precipitation hardening and are customarily employed in the precipitation or age-hardened condition. The optimum solution treatment consists in heating to a temperature close to the solidus of the steel, viz., about 2250 F. or higher, and then cooling fairly rapidly. For elevated temperature service it is customary to age the steel at about the temperatures encountered in service.

The alloys available at present for high temperature service are inadequate because those that do not contain appreciable amounts of cobalt and columbium are limited to service at temperatures no higher than about 1300 F.; whereas those that can withstand high stresses at temperatures over 1300 F. contain large amounts of the strategic elements, cobalt and columbium. Since the latter are in short supply and would not be readily available in time of war, it is desirable to have alloys of high strength which are serviceable at temperatures at least up to 1500 F., and which contain little, or none, of these elements.

Of those steels free from appreciable amounts of cobalt and columbium which are in use at present, most have to be given some special forging treatment in order to develop the maximum high temperature strength properties in the steel. This is a hardship in the fabrication of articles made from these steels because of the need of special control of temperature; limitation of size; and unusual risk of damage to forgings, and to forging equipment.

It is the 'object of this invention to provide forgeable alloy steels containing no cobalt, and only small amounts of columbium, that is, less than about 1%, which have high strength at 1500 F. It is a further object of this invention to provide steels which develop their high temperature strength properties by heat treatment rather than by special forging practice.

The steels of this invention are balanced in composition so that they will be substantially austenitic in structure since it is well known that austenite is much stronger than ferrite at high temperatures. The element most effective in establishing a stable austenitic structure in steel is nickel and therefore the steels of this invention contain upwards of 20% nickel. The upper limit of nickel is set at about 30% because the precipitation hardening reaction in the steels of this invention becomes less effective as the nickel'exceeds this amount.

Since the steel to be useful at temperatures as high as 1500" F. must be resistant to oxidation, an addition of upwards of 16% chromium is made for this purpose. Chromium is also effective in improving the high temperature strength of the steel but amounts over about 22% cause thesteel of this invention to be very difficult-to forge, and therefore the upper limit of chromium is set at about 22%. Furthermore the chromium in the steel is always less than the nickel to maintain the austenitic structure in the steel and avoid the formation of delta ferrite.

Carbon is present in the steel. for thepurposes of stabilizing the austenitic structure, and. contributing to the strength of the steel at high temperatures; 0.30%, because when the high-temperature, strength-giving elements, tungsten and molybdenum, are present in the steel in proper amounts, the steel becomes very difficult to forge when the carbon is present in amounts over about 0.30

Manganese and silicon are both present in the steel in usual amounts. for the purposes of deoxidation' of the steel and improving the forgeability; Manganese: is useful up to about 2.0% r

and silicon up to about 1.0%.

As is well known, av steel that is merelyaustenitic in structure is not capable of supporting much stress at temperatures as high as 1500 F; For example Table 1 shows the stress values which cause rupture in. 100 hours for three of the well known austenitic steels, Type 302' (18-8.) Type 316 (18 -13-3):; and Type 310 (25-20). Along with. these. are shown values published. by Grant, Frederickson, and Taylor. in March 18 and April 8,1948 issues of. Iron Age for some modern high strength alloys;

TABLE I.

but the carbon is. limited to about.

amounts of tungsten, or molybdenum, and preferably a mixture of the two.

That the amount of tungsten and molybdenu required is critical is indicated in Table II bel w, which shows the time-to-rupture for a serie of steels at 1500" F1. under'a stress of 22.000 p. s, i, Since these are precipitation hardening st m they were solution treated at 2300 to 2350 Fj,f 1 lowed by an air cool, and then aged at 1500 for about hours. prior to being tested.

TABLE II Time-to-rupture. at 1500 F. with stress of 22,000 p. s. i.

All steels contain. .10/.20 C'; 1.0/ Mn; .10/1.0 Si; 22.0/300 Ni; and

. W plus Hours to Steel W Mo M o Rupture GO'l 0.2 (30-2 2.5 2.5 0.3 GO 3..- 4.3 4.3 0.2 GO 4... 6.2 6. 2. 1' GO 5..- 6.2. 1.3 7.5 2' GT 1..-

8.4 8.4 3 GO 6.... 1.6.2 31 3, 9f5 0 GT 2 3.0 7.1. 10.1 ll GT 16 10.2 0.2- 10.4 10' GT 4;. 5.4. 5. 5' 10.9v GT 3 6.5- 5.8 12.8 3]. GT' 6 8. 5 4. 2 l2 .'7 GT 1 10.3 2.0 13.2 21 GT 20 13.2 0.3. 13.5. 19 GT 51"-. I 8.7 5.4 14:1 G1 2 l 14.5 0.2. 14:.7 30 GT. 18 10.7 4. 8 15..5 3 9 GT- 21 13, 2 2. 8 1610 '27 GT-lQ 10.4 6..=7 17;l 36; GT'22 13.1 4.9. 18.0 39 GT 25 15. 5' 3. 0 18:5" 23' The data of Table II"indicate-that a of about 10% of tungstenplus molybdenum; is

Stress tocause: rupture in iflo hours'at. 1500 12,.

for various: known alloys- Analysis per cent Grade" Norm: Manganese 2% max. in all of the above alloys.

It will, benoted thatof. thosealloyswhich-have. valuesof. str.ess over.15.000 p. .s. i. for. a 10.0 hour. rupturetimeat1500 F; S.495..contains- 4%.colum bium;. N. 15.51 and. S; 590 contain. 20% cobalt plus,- 1% and/1%..columbium,,respectively; and. S 816 contains. cobalt. lus, 4%. columbium, It. is. evident from these dataandothers. published in the IronAgearticlereferred to .above, thatknown alloys which will withstand..a..stress of morethan 15,000'p. s. i. at 1500'? F. for hours must have appreciable: amounts. of either cobalt, or columbiunr (or tantalum) or both, in their composition.

The steels'ofthis invention, on the other. hand, have-stress values" over 15,000 p. s. i., and ashigh as 20,000-p:- s. i: fora rupture time of 100 hours at 1500 "F.'.', and these steelscontain no cobalt, and-although they-- may contain columbium up to; about 1%, their high stress values: are not dependenteta anylarge' degree on thle presenoe of' columbium in the steel. The high; strength of:

these steels is dependent on the presence 91 si able required for the steel to have-any appreciable lite.- under'thesetest conditions. On the other'hand, although there is some further increase in. life as the sum of tungsten plus molybdenum increases from about 10 to about 20%, the magnitude of change is by no means as. great. as. it from 0 to about 10% of these two elements. Further? more; the forgeabi'lity of the steel definitelyrde teriorates asthe sum of tungsten and molybdenum approaches 20%. It. is desirable: therefore. that the steel containfrom about 10 to.20% of. tungsten plus molybdenum with a minimum of about 3%- for either element, and a. maximum about-15 of; either.

These alloys are precipitation hardening and.- thezoptimum' solution treatment. consists of heating to a temperaturecl'ose. to: the solidus of. the.

steeLabout 22.50.? H. or highenzand then cooling;

nerses 01' these steels is eleigated temperature service it is customary to age: the steel at about the temperatures encounteret l in service.

TABLE III shown in Table III. For

of this invention are shown in Table IV.

6 TABLE v and 1350 F.

Samples solution treated at 2300 F. and aged at 1400 F.

for 15 hours.

.E'fietct of. aging treatment on room temperature g hardness Stress p. s. 1. to Cause Rupture All ste eis contain about .10 .20 C;1.0/2.0 Min; .10 1.0 Si; 25/30 Ni; 17/21 0 c 0r; besidesW and Mo. l l M 1200 F at 1350 F All s mples solution treated at 2 350 F, water quenched then aged at ii ndi'cated temperatures for times shown. 100 hr 1000 hr 100 1000 hr 4' v Rmkweno Hardness 50, 000 40,000 32, 000 20,000 Steel W Mo s. T.1 11300F. 11400F. 1500 F.

What I claim is:

: 3 a4 3 s 4 3 3 3 1. A forgeable, austenitic, alloy steel, characterized in having a 100-hour stress-to-rupture s705 5.0 5.3 11 2s 30 as 28 3s 30 2s 33 33 V lue at 1500 F. of at least 15,000 po nd P 1 18 34 44 47 41 44 45 41 41 41 square inch (p s i) said steel containing about 8706.. 11.2 5.0 11 17 as 44 as 42 43 40 40 41 20 20 to 30% nickel and about 16 to 22% chromium, 1S Trmrdness as Solution man the chrommm content being less than the nickel Aging temperature. content; about 3 to each of tungsten and Aging time, hrs. molybdenum, the combined content of these two Some stress rupture values at 1500 F. of steels 25 elements being about 10 to carbon from an eifective amount up to about 0.3%; manga- TABLE IV Stress rupture values of alloys of invention at 1500 F.

All test pieces solution treated at 2,300-2,350 F. and aged for 16 hours at 1,500 F.

Stress to Rupture (p. s. i.) at l,500 F. Steel 0 Mn Si Ni Cr W Mo V Cb T1 in 100 in 1,000 hrs. hrs. 1

GO 6 09 1. 2 26. 6 18. 8 6. 2 3. 3 14, 000 11, 500 GT 2.- 11 1. 3 15 27. 5 18. 8 3.0 7. l 000 14, 300 GT 4 l2 1. 5 27. 5 l8. 1 5. 3 5. 4 18, 000 14, 500 GT 3"--- .11 l. 4 15 27. 5 19. 1 6. 5 5. 8 20, 000 16, 000 GT 6.--" 09 l. 5 30 27. 9 18.0 8. 4 4. 2 19. 000 15, 500 GT 5 .11 l. 5 .30 27. 5 17.8 8. 6 5. 4 19, 000 14, 500 GT l2 12 1. 3 22. 8 19.5 7. 0 5. 3 20, 500 17, 000 GT l3. 13 1.3 .45 25. 4 l9. 3 7. l 5. 4 19, 500 15, 000 GT 10---- 29 1.3 .45 27. 7 19. 3 6.9 5.3 20, 000 10, 000 GT 7 16 1. 3 30 27. 7 18.4 6. 6 5. 7 19,000 15, 500 GT 29-. .15 1.3 .35 26.4 19.0 5.9 5.7 000 .000 GT 30.--- 15 1.3 35 26. 2 18.2 5. 8 5. 8 18, 500 16, 000 GT 31".- 14 1.4 26. 5 18.9 6.1 5. 4 19, 500 17, 000 GT 32- 17 1. 5 35 28. 5 19.7 6. 3 5.8 20, 000 17, 500

1 Extrapolated from log-log plots of stress vs. time-to-rupture values.

15,000 p. s. 1., whereas all the other steels with values of the sum of tungsten and molybdenum over 10% have 100-hour values at 1500 F. well over 15,000 p. s. i. It will be noted also that the steels having a sum of tungsten and molybdenum over 10% have 100 hours stress rupture values at 1500 F. equal to, or better than, those of all the alloys listed in Table I, with the exception of S 816, which contains as much as 45% and 4% of cobalt and columbium, respectively. It will also be noted that although additions of V, Cb, and Ti in the steels of this invention in amounts up to about 1 do not much afiect the 100-hour rupture value at 1500 F., they generally do improve the 1000-h0ur'value at 1500 F., therefore these additions are useful for improving the long time life "of the steel of this invention under stress at nese up to about 2%; silicon up to about 1%; vanadium up to about 3% titanium up to about 2%; columbium up to about 1%; and the balance Iron.

2. An age-hardened alloy steel, characterized in having a -hour stress-to-rupture value at 1500 F. of at least 15,000 p. s. i., said steel containing: about 20 to 30% nickel and about 16 to 22% chromium, the chromium content being less than the nickel content; about 3 to 15% each of tungsten and molybdenum, the combined content of these two elements being about 10 to 20% carbon from an efiective amount up to about 0.3%; manganese up to about 2%; silicon up to about 1%; titanium up to about 2%; columbium up to about 1%; vanadium up to about 3% and the balance iron.

3. A substantially cobalt free, austenitic and forgeable alloy steel, characterized in having a 100-hour stress-to-rupture at 1500 F. of at least 15,000 p. s. i., said steel containing: about 20 to 30% nickel and about 16 to 22% chromium, the chromium content being less than the nickel content; about 3 to 15% each of tungsten and molybdenum, the combinedjcontent of these two elements being-about 10 to 20%; carbon from an e'fie'ctiv'e amount up to fabout 0.3%; 'up to about 1% columbium; up to about 10% in aggregate of other elements which do not impair the high strength, high temperature characteristics of the steel, and the balance iron.

4. A substantially cobalt free, age-hardened alloy steel, characterized in having a 100-hour stress-to-rupture value at 1500 F. of at least 15,000 p. s. i., said steel containing: about 20 to 30% nickel "and about 16 to 22% chromium, the chromium content being less than the nickel content; about 3 to each of tungsten and molybdenum, the combined content of these two elements being about 10 to 20%; carbon from an effective amount up to about 0.3%; up to about 1% columbium, up to about 10% in aggregate .of

'other-=elements which do not'impair the high strength, high temperature characteristics of the steel, and the balance iron.

5. A iorgeable alloy steel, characterizedin having a l'00-hour stress-to-rupture at 1500 F. of at least 15,000 p. s. i., said steel containing: about 22 to nickel; about 16'to 22% chromium; about 3 to 8% each of tungsten and mo1ybdenum, the combined content of these two elements being about 10 to 16%; about 1 to 2% manganese; about 0.2 to 0.5% silicon; about 0.1 to 0.2% carbon; and the balance substantially all iron.

REFERENCES CITED The following references are of record in thle file of this patent:

UNITED STATES PATENTS Number Name Date 2,125,929 Krivobok Aug. 9, 1938 2,403,128 Scott et a1 July 2, 1946 FOREIGN PATENTS Number Country Date 461,251 Great Britain Feb. 10, 1937 OTHER REFERENCES Patented file of Patent 2,125,929, issued August 9, 1938, paper No. 6, pages 1 to 4. 

1. A FORGEABLE, AUSTENITIC, ALLOY STEEL, CHARACTERIZED IN HAVING A 100-HOUR STRESS-TO-RUPTURE VALUE AT 1500* F. OF AT LEAST 15,000 POUNDS PER SQUARE INCH (P. S. I.), SAID STEEL CONTAINING: ABOUT 20 TO 30% NICKEL AND ABOUT 16 TO 22% CHROMIUM, THE CHROMIUM CONTENT BEING LESS THAN THE NICKEL CONTENT; ABOUT 3 TO 15% EACH OF TUNGSTEN AND MOLYBDENUM, THE COMBINED CONTENT OF THESE TWO ELEMENTS BEING ABOUT 10 TO 20%; CARBON FROM AN EFFECTIVE AMOUNT UP TO ABOUT 0.3%; MANGANESE UP TO ABOUT 2%; SILICON UP TO ABOUT 1%; VANADIUM UP TO ABOUT 3%; TITANIUM UP TO ABOUT 2%; COLUMBIUM UP TO ABOUT 1%; AND THE BALANCE IRON. 